Tailoring a local acid-like microenvironment for efficient neutral hydrogen evolution

Electrochemical hydrogen evolution reaction in neutral media is listed as the most difficult challenges of energy catalysis due to the sluggish kinetics. Herein, the Ir-HxWO3 catalyst is readily synthesized and exhibits enhanced performance for neutral hydrogen evolution reaction. HxWO3 support is functioned as proton sponge to create a local acid-like microenvironment around Ir metal sites by spontaneous injection of protons to WO3, as evidenced by spectroscopy and electrochemical analysis. Rationalize revitalized lattice-hydrogen species located in the interface are coupled with Had atoms on metallic Ir surfaces via thermodynamically favorable Volmer-Tafel steps, and thereby a fast kinetics. Elaborated Ir-HxWO3 demonstrates acid-like activity with a low overpotential of 20 mV at 10 mA cm−2 and low Tafel slope of 28 mV dec−1, which are even comparable to those in acidic environment. The concept exemplified in this work offer the possibilities for tailoring local reaction microenvironment to regulate catalytic activity and pathway.


Point-by-point response to the reviewers' comments
We sincerely thank the reviewer's for their careful review on our manuscript and their valuable comments and suggestions, which certainly help to improve our manuscript. We have revised and supplied a lot of experimental data and the corresponding explanations for improving our manuscript. All the changes have been marked in blue and highlighted in yellow in the revised manuscript and supplementary information. The point-by-point responses are presented below.
Comments by Reviewer's: Reviewer #1 (Remarks to the Author): Title: Tailing a local acid-like microenvironment for efficient neutral hydrogen evolution. In the manuscript by Xiaozhong Zheng et al., a novel Ir-HxWO3 catalyst was synthesized and showed outstanding performance for HER in neutral media. By spontaneously injecting protons into WO3, HxWO3 was used as a proton sponge to create an acidic microenvironment around Ir sites.

Response：
We express our sincere gratitude to the reviewer for your careful review on our manuscript and the constructive comments, which really help to improve the quality of our manuscript. Following the reviewer's suggestion. we have supplied a lot of experiment data to extent "proton sponges" effect of HxWO3 support to other catalytic systems (M-HxWO3, M = Pt, Ru, Pd, Ni, Co) for developing efficient neutral electrocatalysts, further proving the universality of the concept proposed by this paper.
In addition, we have highlighted differences from other works in the revised manuscript to further demonstrate the quality and depth of this work. In general, we have made great changes to the manuscript according to the suggestions of reviewer, and we hope that our changes will satisfy the reviewer. Next, we will reply to the reviewer's comments one by one. All changes have been highlighted in the revised manuscript and supplementary information files as well.
Q.1 In order to take advantage of Hads' suitable adsorption energy on the cathode, the hydrogen evolution reaction (HER) takes place via a two-electron transfer process involving catalysts containing precious metals such as Pt and Pd. Could other precious 2 metals have been used instead of iridium by the authors?

Response：
We appreciate the reviewer's meaningful suggestions. Inspired by the local acidlike microenvironment created by HxWO3, it is expected to extend "proton sponge" effect of HxWO3 support to other M-HxWO3 systems (M = Ru, Pt, Pd, Co, Ni). As expected in Figure   Pd) were synthesized by the same approach as the preparation of Ir-HxWO3, except that metal precursor was changed to ruodium chloride, chloroplatinic acid or palladiump chloride, respectively. Commercial 20 wt% Pt/C, 5 wt% Ru/C and 5 wt% Pd/C were used as comparison samples. M-HxWO3 (M = Ni and Co) were prepared as following steps. 400 μL 20 g L -1 nickel nitrate or cobalt nitrate was dropped onto 1×1 WO3/CFP, and then dried up. The resulting material was reduced in tube furnace at 500 o C for 3 h in H2 atmosphere (flow rate: 50 sccm). The same approach was conducted on pure CFP to obtain Ni and Co comparison samples. " Q.2 With tungsten oxide as support, Chunlei Peng and colleagues (Appl. Catal. A, 623, 2021, 118293) have already reported improving HER performance by using Ir/W18O49 nanowire catalysts. The authors should shed light on how this work differs from the reported work. Hence, in the present form I do not recommend the publication of this paper.

Response：
We sincerely thank the reviewer for the insightful comments. We have carefully read the paper (Appl. Catal. A, 623, 2021, 118293) provided by reviewer. Next, we will further emphasize the high quality and depth of this work by describing the differences between our work and the article on aspect of several perspectives, the special statements are as follows: 1) The determination of local acid-like microenvironment on HxWO3 support in neutral media. 4 We used rotating ring-disk electrode (RRDE) technique to quantitatively detect local pH on the HxWO3 cathode surfaces at different applied potentials in neutral 1.0 M PBS solution (bulk pH is 7.03), along with classical carbon support for comparison , details see Methods). On the basis of RRDE detecting method, we found that the pH of the HxWO3 surface varies from 6.27 to 3.53 as potential decreases from 0.1 to -0.4 VRHE, which undoubtedly confirms that HxWO3 acts as proton sponge to form a local acid-like microenvironment on the electrode surface ( Fig.   2a,b). In sharp contrast, the pH of carbon cathode surface is maintained at ~7 in range of 0.1 ~ -0.4 VRHE (Fig. 2b). As the potential continued to shift negatively, the pH of HxWO3 and carbon support surfaces gradually increase and approach to 8.32 and 8.0 at -0.7 VRHE, respectively, due to the consumption of the local hydrogen species.
In Figure R4a and 4d, 10% Ir-W18O49 only provides a current density of nearly 30 mA/cm -2 at -0.2 VRHE, while the current density of the Ir-HxWO3 catalyst is boosted by 11 times to achieve ~ 360 mA/cm 2 under the same potential conditions. Tafel slope plots further elucidate that Ir-HxWO3 (28 mV/dec) has faster kinetics compared with 10% Ir-W18O49 (66 mV/dec) in neutral-pH media ( Figure R4b and 4e). With regards to catalytic stability ( Figure R4c and 4f), amazingly, Ir-HxWO3 catalyst exhibits excellent operation stability at 10 and 500 mA cm -2 over 100 h and 40 h, respectively, outperforming 10% Ir-W18O49, commercial Pt/C and most of the recently reported landmark catalysts (Supplementary Table 2). In summary, compared with 10% Ir-W18O49, the compelling activity and stability of Ir-HxWO3 catalyst make it more promising for applications in mild energy storage and conversion systems and direct seawater electrolysis for hydrogen production.  (a-c, Appl. Catal. A, 623, 2021, 118293) The LSV, Tafel slope and stability test of 10% Ir-W18O49 and associated catalysts; (d-f, this work) The LSV, Tafel slope and stability test of Ir-HxWO3 and associated catalysts.

3) Exploration of the catalytic mechanism of neutral HER.
Although much works have been done on tungsten oxide-supported nanometallic catalysts as excellent catalysts for energy conversion (ACS Appl. Mater. Interfaces 2017, 9, 31794;ACS Appl. Mater. Interfaces 2020, 12, 25991;J. Mater. Chem. A, 2019, 7, 6285;Small, 2021, 17, 2102159;Appl. Catal. A, 623, 2021, 118293;Appl. Catal. B Environ., 2021, 296, 120359 and so on), the improved catalytic performances are simply attributed to the electronic regulation and synergistic catalytic effect. The exact catalytic mechanism is unclear, especially in challenging neutral media. There still lacks direct experimental evidence to prove the promoting effect of HxWO3 support in neutral HER. In this work, the solid experimental evidences that including operando Raman measurements, selective poisoning and kinetic isotope effect experiments confirm the coherent synergism between Ir and local Brønsted acid W-OH species of HxWO3 for the first time, that is, Volmer process (water dissociation step) is drastically boosted at Ir site to form Ir-H*, followed by spontaneous recombination of Ir-H* and neighboring revitalized WO-H* species to form H2 molecular via interfacial Tafel step, as verified by theoretical calculations, thereby keeping the reaction at a high rate as observed in HER performance tests. 7 4) The extension of "proton sponges" effect of HxWO3 support.
In our work, the "proton sponge" effect of HxWO3 support can be universally transferred to other M-HxWO3 systems (M = Ru, Pt, Pd, Co, Ni). Compared with traditional M-Carbon systems, the significantly reduced overpotentials and accelerated reaction kinetics on M-HxWO3 systems (M = Ru, Pt, Pd, Co, Ni) confirm that the local acid-like microenvironment provided by HxWO3 fundamentally improve the intrinsic HER activity of catalysts in thermodynamically unfavorable neutral media.

5) Neutral water electrolysis device performance.
To highlight the practical significance of localized acidification environmental engineering for neutral water reduction, we further integrated bifunctional Ir-HxWO3 catalysts into a membrane electrode assembly (MEA) as cathode and anode materials and assembled an actual anion-exchange-membrane water electrolysis device ( Figure   R5 and Fig. 8a, details see Methods). The current density of the MEA composed of Ir-HxWO3/CFP(±) is much higher than that of the MEA composed of benchmark commercial (-)Pt/C + Ir/C(+) under the same cell voltage. At a current density of 10 mA cm -2 , the cell voltage is 1.78 V for Ir-HxWO3/CFP(±)-based MEA system, which is much less than that of 2.05 V for benchmark commercial (-)Pt/C + Ir/C(+)-based MEA setup (Fig. 8b). Significantly, the Ir-HxWO3/CFP(±) MEA can be stably operated for at least 40 h at a larger current density of 150 mA cm -2 (Fig. 8c), demonstrating unprecedented application prospects.
8 Figure R5 and revised Fig. 8: Neutral water electrolysis device performance. a, Photographs and schematic illustration of membrane electrode assembly (MEA) electrochemical reactor, the geometric area of the electrode is 4 cm 2 . b, Neutral water splitting performance of the commercial (-)Pt/C + Ir/C(+) and Ir-HxWO3/CFP(±) MEA setups at room temperature. c, Stability tests of the Ir-HxWO3/CFP(±) MEA. Accordingly, the above highlighted text and corresponding figure are added to the Main text of the revised manuscript (in Page 23-24, Line 440-456).

Special changes in Introduction section are as follows:
In Page 3, Line 48: "Therefore, selecting a suitable system to create a local acid-like environment through multiple physicochemical effects to the maximum extent possible, will provide an alternative way to promote the electrocatalytic performance and guide the higher efficiency electrocatalyst design in non-acidic electrolyte, especially in more challenging neutral media." In Page 3, Line 60-65: "Up to now, it still encounters many problems and challenges, for example, 1) the degree of local acidification of HxWO3 in neutral media has not been accurately quantified; 2) the synergistic catalysis between local acidic species and co-catalysts has not been fully understood; 3) the enhanced activity cannot be simply attributed to a 9 single optimized catalytic site, and the origin of the activity deserves further investigation." Minor: Pg. No. 3, line 55, "as as a proton" replace with "as a proton" Pg. No. 3, line 55, "a acid-like" instead of "an acid-like" Pg. No. 22, line 426, "graphite rod"-should check font size.

Response：
We thank the reviewer for the useful suggestions. First of all, we are sorry for our carelessness. According to the kind reminders from reviewer, we have made corresponding changes in the revised manuscript, the details are as follows: In Page 3, line 55: "Protonated HxWO3 could act as a proton sponge and electron reservoir to create an acid-like microenvironment in the electric double layer, thereby further affecting the reaction barriers and pathway [34][35][36] ." In Page 26, line 501: "WO3 precursor, a graphite rod and a saturated calomel electrode (SCE) were used as the working electrode (WE), counter electrode (CE) and reference electrode (RE), respectively." Reviewer #2 (Remarks to the Author): In this work, the authors reported an interesting work of tailing the local catalytic microenvironment to improve the sluggish kinetics of neutral hydrogen evolution reaction (HER). Comprehensive experiments, ex/in-situ characterizations, and DFT calculations were employed to unveil the coherent synergism between Ir and spontaneously injected lattice-hydrogen species of HxWO3.
The Volmer process is drastically boosted at the Ir site to form Ir-H*, followed by thermodynamically favorable recombination of Ir-H* and neighboring revitalized WO-H* species to form H2 molecular via interfacial Tafel step. The Ir-HxWO3 shows exciting neutral HER activity with an ultralow overpotential of 20 mV at 10 mA cm -2 and a low Tafel slope of 28 mV dec -1 , even comparable to those in an acidic environment. In summary, this work provides a novel and vital viewpoint to understand catalytic behavior in electrochemical systems. The work is novel and important to the field. I recommend the acceptance of this work for publication in Nature Communications after the authors address the following revisions.

Response：
We appreciate the reviewer's positive feedbacks and evaluating our work as "novel and important to the field". We also express our sincere gratitude to the reviewer for all the constructive comments and suggestions, which really help to improve the quality of our manuscript. Following the reviewer's suggestions, some extended experiments were conducted to verify the universality of the proton sponge effect of HxWO3 support.
In addition, we have cited some important literature that reviewer recommended and checked the manuscript carefully to avoid language errors. In generally, we hope that our changes will satisfy the reviewer. Next, we will reply to the reviewers' comments one by one. All changes have been highlighted in the revised manuscript and supplementary information files as well.
1. Whether the proposed concept or catalytic systems of this work can be transferred to other metal catalyst systems (such as Pt, Ru, etc.) and achieve similar effects?

Response：
We appreciate the reviewer's meaningful suggestions. Inspired by the local acid-11 like microenvironment created by HxWO3, it is expected to extend "proton sponge" effect of HxWO3 support to other M-HxWO3 systems (M = Ru, Pt, Pd, Co, Ni). As expected in Figure   Pd) were synthesized by the same approach as the preparation of Ir-HxWO3, except that metal precursor was changed to ruodium chloride, chloroplatinic acid or palladiump chloride, respectively. Commercial 20 wt% Pt/C, 5 wt% Ru/C and 5 wt% Pd/C were used as comparison samples. M-HxWO3 (M = Ni and Co) were prepared as following steps. 400 μL 20 g L -1 nickel nitrate or cobalt nitrate was dropped onto 1×1 WO3/CFP, and then dried up. The resulting material was reduced in tube furnace at 500 o C for 3 h in H2 atmosphere (50 sccm). The same approach was conducted on pure CFP to obtain Ni and Co comparison samples. " 2. Ir-based materials show high OER activity in acidic media. So, it would be very interesting if the author could test the OER activity of Ir-HxWO3 in acidic solutions.

Response：
We appreciate the reviewer's meaningful suggestions. According to the reviewer's comments, we test the OER activity of Ir-HxWO3 in acidic and neutral solutions. As shown in the Figure R6, Ir-HxWO3 catalyst gives an extremely low OER overpotential of 247 and 327 mV at 10 mA cm -2 and a Tafel slope of 87 and 70 mV dec -1 in 0.5 M H2SO4 and 1 M PBS, respectively, both significantly lower than those of commercial Ir/C (acidic: 316 mV, 157 mV dec -1 ; neutral: 497 mV, 247 mV dec -1 ). Inspired by reviewer comments, Ir-HxWO3 is expected to act as a "universally compatible" electrocatalyst that simultaneously shows excellent HER and OER performances in neutral condition. To highlight the practical significance of localized acidification environmental engineering for neutral water reduction, we further integrated bifunctional Ir-HxWO3 catalysts into a membrane electrode assembly (MEA) as cathode and anode materials and assembled an actual anion-exchange-membrane water electrolysis device ( Figure   R5 and Fig. 8a, details see Methods). The current density of the MEA composed of Ir-HxWO3/CFP(±) is much higher than that of the MEA composed of benchmark commercial (-)Pt/C + Ir/C(+) under the same cell voltage. At a current density of 10 mA cm -2 , the cell voltage is 1.78 V for Ir-HxWO3/CFP(±)-based MEA system, which is much less than that of 2.05 V for benchmark commercial (-)Pt/C + Ir/C(+)-based MEA setup (Fig. 8b). Significantly, the Ir-HxWO3/CFP(±) MEA can be stably operated for at least 40 h at a larger current density of 150 mA cm -2 (Fig. 8c), demonstrating unprecedented application prospects. "Neutral water electrolysis device. For a neutral water electrolysis device system, the bifunctional Ir-HxWO3/CP catalysts (2×2 cm 2 ) was both for the anodic OER and cathodic HER. As for benchmark commercial (-)Pt/C + Ir/C(+) partners, homogeneous slurries consisting of catalysts, Nafion solution (5.0 wt.%) and ethanol were air-sprayed onto the carbon fiber paper with an iridium black loading of 2.0 mg cm -2 for the anode and 1.0 mg cm -2 of Pt/C for the cathode. In all, 1.0 M PBS electrolyte was cycled both on the anodic and cathodic sides by a peristaltic pump, and the flow rate is 80 mL min -1 . Anion-exchange-membrane (Fumasep FAA-3-PK-130) was used to to isolate the cathode and anode. Polarization curves were collected from 1.0 to 3.5 V at room temperature under an ambient pressure. The current density was calculated against the geometric area (4 cm 2 ) of the MEA to obtain the specific activity without iR 15 compensation. The stability test was carried out by galvanostatic electrolysis at a constant current density of 150 mA cm -2 ." 3. Generally, the Tafel slope for Pt/C in acidic media is about 30 mV dec -1 . In this work, the Tafel slope is ~15 mV dec -1 in Figure 3b. Why? The authors should explain.

Response：
We sincerely thank the reviewer for the insightful comments and we are pleased to clarify this issue. As we all known that, the Tafel equation is of fundamental importance in electrochemical kinetics, formulating a quantitative relation between the current and the applied electrochemical potential. Currently, Tafel plots are often extracted from potential polarization curves (such as linear sweep voltammetry and cyclic voltammetry) in the literature. However, the lack of a standard method for analyzing catalytic performance has prevented researchers from fairly comparing the catalytic properties of different nanomaterials for HER, even for state-the-of-art Pt/C catalytic system (ACS Nano 2018, 12, 9635-9638;ACS Energy Lett. 2021, 6, 1607-1611. Table R2 shows the comparison of the reported Tafel values of commercial Pt/C under acidic conditions. It can be seen that the Tafel values of commercial Pt/C range from 10 to 60 mV dec -1 and are mostly below 30 mV dec -1 ( Figure R7-R10). These results reveal that the Tafel slope is strongly related to the scan rate, catalyst loading, substrate electrode, the selected fitting potential region and even manufacturer.
To address the reviewer's concerns, we measured Tafel slopes of Pt/C on glassy carbon (GC) electrode again and verified the accuracy of our measurement. Firstly, to minimize the possibility of mass transport limitation associated with the reactant and product from the electrode, solution was stirred during the collection of Tafel data.
Secondly, solution resistance was compensated (95%). Thirdly, a slow scan rate of 1 mV s -1 was used to obtain the steady-state collection of current density at each applied potential. In addition, same loading amount of 0.1 mgPt cm -2 was conducted to ensure a high activity. As shown in Figure R11, the Tafel slope of Pt/C is 13.2 mV dec -1 in the range of 1 to 10 mA cm -2 , which is close to that of Pt/C on CFP. Combined with the above discussion, in this work, for the purpose of parallel performance comparison, 16 commercial 20 wt% Pt/C loading on carbon fiber paper was chosen as standard catalytic system.

Response：
Thank you for giving the constructive suggestions. According to the reviewer's comments. We supplied the high-resolution O 1s spectra of Ir-HxWO3 and HxWO3 samples. As shown in Figure   5. In Figure S6, it is not easy to identify the curves. It would be better if the authors use different colors.

Response：
We thank the reviewer for the useful suggestions. Based on the suggestions of the reviewer, we replotted Supplementary Figure 9 with different colors to better present the experimental data.

Special changes are as follows:
In Page S11: 20 Figure R13 and revised Supplementary Figure 9. Activation process of Ir-HxWO3. HER polarization curves of the Ir-HxWO3 after different numbers of potential cycles were performed on it.
6. The Ir-HxWO3 shows outstanding HER activity in neutral conditions with an ultralow overpotential of 20 mV at 10 mA cm -2 , which shows good practical applications, while the stability test in Figure 3h is only nearly 50 h. It would be better if the author can further extend the stability test time for such high-activity materials.

Response：
Thank you for your constructive comments. According to your suggestion, we try to extend the stability time for Ir-HxWO3 catalyst at a constant current density of 10 mA/cm 2 and compare it with catalysts in the literature to highlight the stability of Ir-HxWO3 catalyst. As shown in the Figure R14 and revised Fig. 4h, increasing the constant current (@ 10 mA/cm 2 ) electrolysis time to 100 hours, Ir-HxWO3 catalyst still represents a relatively stable horizontal line, and the overpotential of the catalyst before and after the test only increases by nearly 14 mV. Notably, the neutral HER catalytic stability is also superior to most of the recently reported landmark catalysts (Table R1 and revised Supplementary Table 2), especially under high current electrolysis. Ir-HxWO3 with high activity and high stability opens the door for the deployment of mild energy storage and conversion systems and direct seawater electrolysis for hydrogen production. and commercial 20% Pt/C (@10 mA/cm 2 ) in 1.0 M PBS.

Response：
Thank you for this kind suggestion. We have read these papers carefully, and cited some important papers as 13,19,20] in the revised manuscript. The specific changes are as follows. should be addressed to improve the quality of this manuscript to meet the standard of the prestigious Nature Commun.

Response：
We express our sincere gratitude to the reviewer for all the constructive comments and suggestions, which really helped to improve the quality of our manuscript.
Following the reviewer's suggestions, we determine the Ir loading amount of Ir-HxWO3 and commercial Ir/C catalysts, and have updated the descriptions about Ir crystal structure. In addition, we experimentally and theoretically elucidate why Ir was chosen as the active component in this work. In generally, we hope that our changes will satisfy the reviewer. Next, we will reply to the reviewers' comments one by one. All changes have been highlighted in the revised manuscript and supplementary information files as well.
1. The author claimed that the local hydride WO3 (HxWO3) phase was created during the negative potential-cycling of WO3 in PBS solution. Are there lattice contraction/extention after H insertion into WO3?

Response：
We thank the reviewer for the useful suggestions. As the reviewer mentioned that, the injection of foreign hydrogen atoms inevitably affects the lattice structure of the  seems not to be very low. Therefore, the authors must obtain the Ir crystal pattern from XRD analysis.

Response：
(1) We thank the reviewer for raising the professional question. First of all, we apologize for confusing reviewers with our incorrect descriptions for Ir crystal structure.
In fact, the reviewer is correct. As shown in the Figure R16 and revised Fig. 2c, the fast Fourier transform (FFT) pattern of Ir nanoparticle exhibites (111) and (200) facets, suggesting face-centered cubic crystal structure, which also agree with the results of the literature (Nat. Commun., 2021, 12, 4271;Adv. Funct. Mater., 2022, 32, 2113191;Adv. Mater., 2018, 30, 1805606;Appl. Catal., B, 2019, 258, 117965) and crystal library data (https://materialsproject.org/). We accordingly modified Fig. 3c and  (2) According to the review's suggestions, we determined the loading amount of Ir on HxWO3 support with inductively coupled plasma-optical emission spectrometry (ICP-OES). The specific results are shown in the Table R3 and revised Supplementary   Table 1. (3) We provide additional larger-scale TEM images to reflect the distribution of Ir NPs in HxWO3 nanorods support. As displayed in the typical TEM images of Ir-HxWO3 at different locations ( Figure R17), Ir NPs are sparsely loaded on HxWO3. Based on that, the XRD signals of Ir NPs are not detected, possibly due to their small sizes (~ 1.7 nm) and low content (2.8 wt%), which is consistent with noble-metal-supported 31 catalysts in the reported literature (ACS Catal., 2017, 7, 7131;Energy Environ. Sci., 2018, 11, 800;Angew. Chem. Int. Ed., 2023, 62, e2023022). In order to further eliminate the doubts of reviewer, different Ir loading amount (3, 5, 10 and 20 wt%) on HxWO3 support catalysts were prepared to obtain their XRD patterns. As we can see in the Figure R18, the characteristic diffraction peak of metal Ir (PDF#02-1155) did not appear until the Ir loading reaches 20%.

Response：
We sincerely thank the reviewer for the insightful comments and we are pleased to clarify this issue. Then, the detailed interpretations are presented from both the experimental and theoretical aspects as listed below:

1) From theoretical aspect:
Iridium (Ir), with its weak hydrogen binding energy, was first selected for the chemisorption of hydrogen. In a hydrogen adsorption/desorption test, Ir with a specific (111)

2) From experimental aspect:
Furthermore, we synthesized Pt-HxWO3 and Ru-HxWO3 catalysts and compared them with Ir-HxWO3, respectively. As shown in the Figure R20, Ir-HxWO3 catalyst exhibits lowest overpotentials at different current densities and significantly expedited reaction kinetics. More importantly, in addition to catalytic activity, the superior stability (100 h@10 mA cm -2 with Δη = 14 mV) makes Ir-HxWO3 catalysts more promising for industrial applications compared with Pt-HxWO3 (40 h@10 mA cm -2 with Δη = 132 mV) and Ru-HxWO3 (48 h@10 mA cm -2 with Δη = 76 mV). Additionally, Ir-HxWO3 catalyst gives an extremely low OER overpotential of 327 mV at 10 mA cm -2 ( Figure R21) and a Tafel slope of 70 mV dec -1 in and 1 M PBS, both significantly lower than those of commercial Ir/C (497 mV, 247 mV dec -1 ). Therefore, Ir-HxWO3 is expected to act as a "universally compatible" electrocatalyst that simultaneously shows excellent HER and OER performances in neutral condition.  To highlight the practical significance of localized acidification environmental engineering for neutral water reduction, we further integrated bifunctional Ir-HxWO3 catalysts into a membrane electrode assembly (MEA) as cathode and anode materials and assembled an actual anion-exchange-membrane water electrolysis device ( Figure   R5 and Fig. 8a, details see Methods). The current density of the MEA composed of Ir-HxWO3/CFP(±) is much higher than that of the MEA composed of benchmark commercial (-)Pt/C + Ir/C(+) under the same cell voltage. At a current density of 10 mA cm -2 , the cell voltage is 1.78 V for Ir-HxWO3/CFP(±)-based MEA system, which 35 is much less than that of 2.05 V for benchmark commercial (-)Pt/C + Ir/C(+)-based MEA setup (Fig. 8b). Significantly, the Ir-HxWO3/CFP(±) MEA can be stably operated for at least 40 h at a larger current density of 150 mA cm -2 (Fig. 8c), demonstrating unprecedented application prospects. along with classical carbon support for comparison. In addition, we have highlighted differences from other works that the reviewer provided to further demonstrate the novelty and depth of this work. In general, we have made great changes to the manuscript according to the suggestions of reviewer, and we hope that our changes will satisfy the reviewer. Next, we will reply to the reviewers' comments one by one. All changes have been highlighted in the revised manuscript and supplementary information files as well.
(1) The catalyst of Ir-HxWO3 reported in this manuscript is similar to the previously reported noble metal-WO3 HER catalyst, which reduces the novelty of this article

Response:
We sincerely thank the reviewers for their comments on this work and we are happy to reemphasize the high quality and depth of this work by describing the differences between our work and previous works provided by reviewer on aspect of 38 several perspectives, the specific statements are as follows: (1) Nano Energy 71 (2020) 104653.

Differences in electrolyte systems.
Wang et al. controllably synthesized a series of tungsten oxides loaded platinum catalysts (Pt-WO3) with excellent HER activity closing to that of commercial Pt/C in acidic media (0.5 M H2SO4 solution). It is well accepted that the presence of a large number of H3O + species in the acidic solution allows the Volmer step to proceed smoothly on most of the catalyst surfaces (M-Hads), followed by hydrogen generation via Heyrovsky or Tafel step. The metallic platinum (Pt) is still considered as "the Holy Grail" of HER electrocatalyst in acidic media with a nearly-zero onset overpotential and fast kinetics owing to its favorable hydrogen binding energy. Excellent catalytic activity is usually obtained with Pt-based catalysts under acidic media (Angew. Chem. Int. Ed., 2023, 135, e202300;Appl. Catal., B, 2022, 314, 121503;J. Electroanal. Chem., 2021, 896, 115076 (2) Nat. Commun., 2022Commun., , 13, 2024Nat. Commun., 2019, 10, 4876. First of all, we would like to thank the reviewers for providing us with these two high-quality papers, which have made an indelible contribution to the regulation of the where the ionic conductivity is much worse. In addition, the limited degree of acidification greatly obstacles its stable operation under high current conditions. In our work, WO3, which has a strong proton storage capacity and excellent proton conductivity, was chosen as the support to achieve the maximum local acidification nearby the metal sites under neutral conditions ( Figure R23). Ir-HxWO3 exhibits highly competitive catalytic activity at high current density, reaching ~360 mA cm -2 at a potential of -0.2 VRHE and operating stably for 40 h at an industrial-grade current density of 500 mA cm -2 .

Special changes are as follows:
Introduction section. In Page 3, Line 48: "Therefore, selecting a suitable system to create a local acid-like environment through multiple physicochemical effects to the maximum extent possible, will provide an alternative way to promote the electrocatalytic performance and guide the higher efficiency electrocatalyst design in non-acidic electrolyte, especially in more challenging neutral media."

In Page 3, Line 60-65
"Up to now, it still encounters many problems and challenges, for example, 1) the degree of local acidification of HxWO3 in neutral media has not been accurately quantified; 2) the synergistic catalysis between local acidic species and co-catalysts has 42 not been fully understood; 3) the enhanced activity cannot be simply attributed to a single optimized catalytic site, and the origin of the activity deserves further investigation." (2) The local acid-like microenvironment proposed by the authors lacks sufficient evidence. What is the pH of the local acid-like microenvironment? Is this local acidlike microenvironment stable during the HER processes? The fugacity of H2 is assumed to be equal to unity and R, T and F are the gas constant, the absolute temperature and the Faraday constant. Hence, the relationship between the disk pH and the ring pH can be corroborated experimentally.

Response：
To confirm that the OCP of the Pt ring electrode accurately reflects the hydrogen equilibrium potential, the ring OCPs were measured with solutions of various pH values without any reactions at the disk electrode at room temperature (~ 20 o C). The time dependence of the ring OCP is shown in Figure R24b.  Next, the local pH measurement was performed on the HxWO3 and carbon support in neutral pH solution during HER. A solution of H2-saturated 1.0 M PBS was used as an electrolyte. The catalyst loading is 0.51 mg cm -2 . The bulk pH of the solution is 7.03.
To obtain steady-state local pH value, constant potential method was performed on the disk electrode, and OCP was measured simultaneously on the Pt ring electrode. In the constant potential method, each potential is maintained for 200 s ( E = 0.1, 0, -0.1, -0.2, -0.3, -0.4, -0.5, -0.6 and -0.7 VRHE ) to obtain a steady-state current response (j).
All measurements were carried out at rotation speed of 1600 rpm at room temperature.
The relatively smooth i-t curves indicates that the electrode surface is in steady state at different potentials ( Figure R25a and R25b). The measured local pH on cathode surfaces (HxWO3 and carbon) are shown in Figure R25c. For the HxWO3 support, the pH of the catalyst surface will be from 6.27 to 3.53 as the potential is reduced from 0.1 to -0.4 VRHE, which undoubtedly confirms its local acid-like microenvironment. As the potential continues to shift negatively (-0.4 to -0.7 VRHE), HER starts to occur and consumes the local hydrogen species, along with the increase in pH. When the potential reaches -0.7 VRHE (j ~ 12 mA), the pH of the catalyst surface becomes 8.32, typical of an alkaline environment. In sharp contrast, the pH of carbon cathode surface is maintained at ~7 in range of 0.1 ~ -0.4 VRHE. As the potential continued to shift negatively, the pH of the carbon support surface gradually increased to 8.0 at -0.7 VRHE (j ~ 5 mA). Astonishingly, when the bias (-0.7 VRHE) is removed, the surface of HxWO3 and carbon cathodes turn back to steady acidic and neutral states, respectively ( Figure   R25d).  Fig. 5).
The OCP of the Pt electrode would indicate the equilibrium potential of 2H + + 2e -→ H2, which varies with pH according to the Nernst equation: The fugacity of H2 is assumed to be equal to unity and R, T and F are the gas constant, the absolute temperature and the Faraday constant.
For measuring the pH on the electrode surface, the investigated catalyst was loaded onto the disk electrode. The catalyst ink was prepared by ultrasonically dispersing catalyst powder (5 mg Fig. 4-6, details see Methods). On the basis of RRDE detecting method, we found that the pH of the HxWO3 surface varies from 6.27 to 3.53 as potential decreases from 0.1 to -0.4 VRHE, which undoubtedly confirms that HxWO3 acts as proton sponge to form a local acid-like microenvironment on the electrode surface ( Fig.   2a,b). In sharp contrast, the pH of carbon cathode surface is maintained at ~7 in range 47 of 0.1 ~ -0.4 VRHE (Fig. 2b). As the potential continued to shift negatively, the pH of HxWO3 and carbon support surfaces gradually increase and approach to 8.32 and 8.0 at -0.7 VRHE, respectively, due to the consumption of the local hydrogen species.
(3) In the operando electrochemical Raman spectra, the WO-H in Ir-HxWO3 gradually disappeared in the voltage range, and the explanation given by the authors is that the surface WO-H species were depleted during the HER process. However, no obvious performance degradation was found in the long-term stability test, there exists a contradiction.

Response：
We sincerely thank the reviewer for the insightful comments and we are pleased to clarify this issue. A combination of operando electrochemical Raman spectra,  Figure R26 and R27 and recently reported literature (J. Phys. Chem. C, 2014, 118, 1, 494;J. Am. Chem. Soc. 2021, 143, 24, 9236;J. Mater. Chem. A, 2019, 7, 23756;Energy Mater. Sol. Cells 1999, 56, 231;J. Catal. 1970, 17, 359), the demonstrated low barriers of H diffusion with water participation are consistent with the high proton-mobility in surface of WO3 observed in experiment. That is to say, transport of H in HxWO3 cannot 50 be the rate determining step in the catalytic process proposed for Ir-HxWO3 system. To more visually demonstrate the continuous replenishment of hydrogen, chronopotentiometry test at 100 mA cm -2 was measured on Ir-HxWO3 catalyst for 10 s, and then observe the change of electrode before and after removing bias ( Figure R28, Supplementary Movie 4 and 5). Counterintuitively, although the amount of bubbles on the surface of the Ir-HxWO3 electrode significantly reduced after stopping CP test, it continued to produce H2 bubbles continuously for at least 1 minute. Using the commercial Pt/C system as a comparison, there was no continuous H2 production on Pt/C catalyst surface when the bias was removed. In a word, The above discussion and experiments confirm that the unique H compensation mechanism of the HxWO3 support makes the local acidification microenvironment stable and effective.  (4) In the XRD pattern of Ir-HxWO3, Supplementary Figure 14a, why were the peaks of the carbon paper enhanced after the HER stability test?

Response：
We sincerely thank the reviewer for the insightful comments and we are pleased to clarify this issue. After HER stability testing, the XRD pattern of carbon paper is enhanced, which mainly stems from the fact that the surface loose HxWO3 nanorods break away from the whole catalyst skeleton under the continuous and violent impact of H2 bubbles during the stability test, thus exposing the carbon paper substrate ( Figure   R29). The same phenomenon has also been reported in other works (ACS Appl. Mater. Interfaces 2018, 10, 14777;Nanoscale, 2021, 13, 8264;J. Phys. Chem. C 2016, 120, 52 16537; J. Mater. Chem. A, 2019,7, 775;ACS Sustainable Chem. Eng. 2018, 6, 11884;Electrochim. Acta, 2017, 241, 106. and Figure R30). Figure R29. (a and b) The typical SEM images of Ir-HxWO3 before and after stability test. Figure R30. The typical XRD patterns of catalysts before and after HER stability test in reported papers.
In order to more visually describe the structure changes of catalyst before and after stability tests, we slightly modified Supplementary Figure  for Pt/C need to be carried to examine their HER mechanism proposed for HxWO3.

Response：
We thank the reviewer for the helpful suggestions. Based on the the reviewer's comments, Li + and SCNpoisoning experiments for HxWO3 and SCNpoisoning experiments for Pt/C were supplied to support the catalytic mechanism proposed for Ir-HxWO3 systems. As we can see in Figure R31, after Li + poisoning treatment, commercial 20 wt% Pt/C catalyst displays negligible activity decay. In contrast, the 20 wt% Pt/C catalyst undergoes SCNpoisoning treatment and shows a drastic activity decrease due to the strong coordination capability between metal-centered catalytic sites and SCNspecies. For HxWO3 support, after Li + and SCNpoisoning treatments, it shows the opposite activity change trend with Pt/C catalyst ( Figure R32)     According to the Supplementary Figure 22 and 23, Li + and SCNions can selectively poison W-OH and metal sites, respectively (J. Am. Chem. Soc. 2021, 143, 20133-20143;Angew. Chem., Int. Ed. 2020, 59, 8982-8990.). After Li + or SCNpoisoning, the Ir-HxWO3 catalyst exhibits striking HER activity decay (Figure 6a), highlighting the importance roles of W-OH species and Ir metal in HER, which is consistent with the Operando Raman results. Then, the Tafel slopes of Ir-HxWO3 catalyst before and after Li + or SCNpoisoning are compared to analyze the change of the catalytic reaction pathway. As shown in Fig. 6b  Tafel process is advantageously utilized to efficiently combine the interfacial Ir-H* and neighboring reactive WO-H* into H2. Unlike the well-known lattice-oxygen-mediated oxygen evolution reaction, the neutral HER mechanism mediated by lattice-hydrogen has been rarely reported. Typically, Patrik Schmuki et al. reported that (J. Mater. Chem. A, 2020, 8, 22773-22790) hydrogen-rich TiO2 surface not only stabilizes the deposited Ir and weakens its H binding strength to a moderate intensity, but also actively takes part in the HER mechanism by refreshing the Ir catalytic sites near the Ir|H-TiO2 interface, thus substantially promoting H2 generation in acidic media ( Figure R1).  Comments 2 and 3. This group has ever published an article (Nature Communications (2022) 13:5382), very similar to this study in characterization methods, experiments, which significantly decrease the innovation of this study. In their previous article, the 5 authors suggested that the high HER activity of Ru/WO3-x is originated from oxygendeficient WO3-x with a large proton storage capacity, they claim that the protons can be transferred to Ru NPs under cathodic potential, increasing the hydrogen coverage on the surface of Ru NPs in HER. In this manuscript, noble metal Ir was used instead of Ru, however, the authors suggest that the lattice hydrogen react at the interface, which is different from the mechanism they proposed for Ru/WO3-x. The findings of this study are not at all consistent with those of Ru/WO3-x, which suggests that further investigations are required.
Response：We sincerely thank the reviewer for the insightful comments and we are pleased to clarify this issue. Firstly, it is worth stating that the surface/interface chemistry of heterogeneous catalysts is highly sophisticated, and changes in components can significantly alter the catalytic activity and mechanism (Chem. Soc. Rev., 2014, 43, 7870-7886;Chem. Rev. 2018, 118, 4981-5079.). As the reviewer mentioned that, our previous work (Nat. Commun., 2022, 13, 5382) developed an effective strategy to significantly increase the H coverage on the catalyst during HER in neutral environment. The oxygen-deficient WO3-x possesses a large capacity for storing protons, which can be transferred to the surface of Ru NPs under cathodic potential. This hydrogen spillover from WO3-x to Ru changes the RDS of HER on Ru in neutral medium from water dissociation to hydrogen recombination (Heyrovsky step: Had + H2O + e -→ * + H2 + OH -), which greatly improves the HER kinetics (see Figure   R2). This interesting experimental phenomenon and conclusion motivated us to further utilize localized hydrogen species of HxWO3 support to realize thermodynamically more favorable Volmer-Tafel (~ 30 mV/dec) steps (Ru-WO3-x follows Volmer-Heyrovsky steps, ~ 40 mV/dec), thus obtaining acid-like HER activity and kinetics in challenging neutral media. Additionally, the interaction mechanism between metal and localized acidic species is still unclear, which is worthy of further exploration. In order to achieve these goals, the composition as well as the structure of the catalyst should be designed rationally. Iridium (Ir), with its weak hydrogen binding energy, was first selected for the chemisorption of hydrogen and exhibits relatively balanced hydrogen adsorption/desorption capacity, comparable to Pt(100)( J. Chem. Phys. 1987, 87, 3104 Environ. Sci., 2020, 13, 3185). Compared with Pt, higher oxygenophilic properties of Ir confer a lower water dissociation energy barrier, thus accelerating Volmer step. In summary, the moderate hydrogen adsorption strength and strong oxygenophilic features of Ir make it a promising candidate for neutral HER catalysts and can be adopted as an ideal model to study catalytic mechanism during the HER process. In this work, a novel Ir-HxWO3 catalyst is readily synthesized. Elaborated Ir-HxWO3 demonstrates acid-like activity and kinetics with ultralow overpotential of 20 mV at 10 mA cm -2 and low Tafel slope of 28 mV dec -1 , which follows thermodynamically favorable Volmer-Tafel steps. To the best of our knowledge, this remarkable performance (overpotential and Tafel slope) is leading among the current neutral HER catalysts (Supplementary Table   2). A combination of physicochemical characterization and theoretical simulation confirm the new interfacial hydrogen-evolution pathway mediated by lattice-hydrogen of HxWO3 (detail analysis see the responses to question 1) and reveal the interaction mechanism between metal and localized acidic species.
To further eliminate the concerns of reviewers, the temperature programmed desorption (TPD) of H2 was conducted to gain further insight into H2 desorption sites in catalysts. As shown in Figure R3, The H2-TPD profile of commercial Ir/C catalyst (microstructure see Figure R4) exhibits a single desorption peak centered at 127 o C, which is ascribed to the desorption of atomic H over the metallic Ir surface. As we expected in Ir-HxWO3 sample, in addition to the observation of H2 desorption site located on the metal surface, the unique dehydrogenation signal over interface of Ir metal and HxWO3 support (155 o C) is also detected, which is consistent with previous results in the literature (J. Catal., 2008, 260, 141-149;Appl. Catal. A-Gen, 2017, 537, 59-65;Top. Catal., 2023, 66, 205-222.). Moreover, by comparing the amount of dehydrogenation at different sites, we found that H2 desorption at the interface site is dominant.